A transmission method according to a discrete multi-tone (DMT) modulation format is used in a metallic line of an access system, for example, a very high bit rate digital subscriber line (VDSL). In recent years, research and development for applying a DMT modulation format to an optical transmission technology is conducted accompanied by increase in network traffic.
A DMT modulation format is a multicarrier transmission technique (see, for example, Japanese Laid-open Patent Publication Nos. 2010-147983 and 2003-332853) based on an orthogonal frequency division multiplexing (OFDM) technology. In the DMT modulation format, data is allocated to each of a piece of subcarriers (SCs) (carrier waves) having different frequencies and allocated data is modulated based on a multilevel degree and a signal power in accordance with transmission characteristics for each SC and is transmitted as a DMT signal. A multilevel modulation format used for data on each SC may include, for example, quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM).